Since the 1800's fingerprint information has been collected from human fingers and hands by means of ink and paper. For purposes of this document, the term “fingerprint” is used to mean the skin surface friction ridge detail of a portion of a hand, such as a single fingerprint, or the entire hand. In recent years various electronic fingerprint scanning systems have been developed utilizing optical, capacitance, direct pressure, thermal and ultrasonic methods. Methods based on ultrasound have proven to be highly accurate, since they are insulated from the effects of grease, dirt, paint, ink and other image contaminants.
In an ultrasonic system, a piezoelectric transducer may be used to send an ultrasonic wave through an ultrasound transmitting media, such as mineral oil. In ultrasonic fingerprint scanners, the ultrasound wave is started and stopped to produce a pulse. At each material interface encountered by the pulse, a portion of the pulse reflects. For example, the interface between a platen and skin or the interface between air and skin may each reflect a portion of the pulse. The fraction of ultrasound reflected is a function of differences in impedance between the two materials comprising the interface. The fraction of ultrasound reflected can be calculated by the equation, R=((Z1−Z2)/(Z1+Z2))2, where R is the fraction of sound reflected, Z1 is the acoustic impedance of the first material and Z2 is the acoustic impedance of the second material. Acoustic impedance is a measure of a material's resistance to the propagation of ultrasound. Acoustic impedance, Z, is defined as Z=r·c, where r is the material density, and c is the longitudinal propagation velocity of ultrasound in the material. The larger the change in acoustic impedance, the larger the fraction reflected.
The reflected wave pulses may be detected by the transducer. The elapsed time during which the pulse traveled from the transducer to the interface and back may be determined. The elapsed time may be used to determine the distances traveled by the pulse and its reflected wave pulses. By knowing the distance traveled, the position of an interface may be determined.
There may be many interfaces encountered by the emitted pulse, and so there may be many reflected wave pulses. Since it is the interfaces associated with a finger that are of interest in generating an image of a fingerprint, it may be necessary to identify those reflected wave pulses that are associated with the finger. The approximate position of a finger being scanned may be known, and therefore the pulse reflected from the finger may be expected during a particular time interval. In a technique commonly referred to as “range gating”, a detector may be configured to ignore reflected pulses that are not received during that time interval. Signals within the expected range may be interpreted to be reflected either from a ridge or a valley of the fingerprint. For example, those signals reflected from a ridge will be received before those signals reflected from a valley. The reflected signals associated with the finger may be processed and converted to a digital value representing the signal strength. The digital value may be used to produce a graphical display of the signal strength, for example by converting the digital values to a gray-scale bitmap image, thereby producing a contour map of the finger surface which is representative of the depth of the ridge structure detail.
Collecting information using an ultrasound transducer is usually accomplished by moving the ultrasound transducer side-to-side while advancing the transducer in a direction that is different from the side-to-side motion. Such an arrangement is commonly referred to as a raster scanning process. As the raster scanning process proceeds, the ultrasound raster scanning mechanism collects each pixel of image information individually, and records those pixels for use in generating an image of the fingerprint.
The prior art scanners are not able to scan more than a small portion of a hand. Since unique identifying features may be found in many areas of the hand, a scanner is needed which can provide information about larger areas of the hand.